Switch arrangement including a thyristor

ABSTRACT

A switching circuit for a flash discharge tube includes a first thyristor in series with the tube across a DC voltage source. The thyristor is rapidly extinguished by an auxiliary circuit that includes, in series, a first capacitor, a second thyristor, and the parallel connection of a second capacitor and a resistor. A charge circuit for the first capacitor is connected partly in parallel with the tube and is exclusive of the first thyristor. A preferred embodiment also includes a diode in shunt with the first capacitor. The novel circuit is arranged to prevent reverse charging of the capacitors so that electrolytic capacitors may be used which results in a compact flash unit.

Roncke June 19, 1973 SWlTCH ARRANGEMENT INCLUDING A THYRISTOR [75]Inventor: Gerhard Ronclte, Hamburg, Germany [73] Assignee: U.S. PhilipsCorporation, New York,

[22] Filed: Mar. 19, 1971 [21] Appl. No.: 126,152

[30] Foreign Application Priority Data Mar. 26, 1970 Germany P 20 14923.9

[52] U.S. Cl. 315/241 P, 315/159, 315/241 [51] Int. Cl. 1105b 41/32,HOSb 41/40 [58] Field of Search 307/305, 246, 252 M;

3,591,829 7/1971 Murata 315/159 X Primary Examiner-Rudolph V. RolinecAssistant Examiner-Marvin Nussbaum Attorney-Frank R. Trifari 57 ABSTRACTA switching circuit for a flash discharge tube includes a firstthyristor in series with the tube across a DC voltage source. Thethyristor is rapidly extinguished by an auxiliary circuit that includes,in series, a first capacitor, a second thyristor, and the parallelconnection of a second capacitor and a resistor. A charge circuit forthe first capacitor is connected partly in parallel with the tube and isexclusive of the first thyristor. A preferred embodiment also includes adiode in shunt with the first capacitor. The novel circuit is arrangedto prevent reverse charging of the capacitors so that electrolyticcapacitors may be used which results in a compact [56] References Citedn h UNITED STATES PATENTS as 3,530,310 9/1970 Adelson et a1. 307/252 M10 Claims, 1 Drawing Figure Paten ted June 19, 1973 y 3,740,610

1 INVLLVTOR.

GE RHAR 0 RONCKE Assn BY 31A $2..

1 SWITCH ARRANGEMENT INCLUDING A THYRISTOR The present invention relatesto an improved thyristor switching arrangement and more particularly toa device for automatically controlling the electronic flash in aphotography system. A device of this type may comprise aswitch-arrangement including a thyristor which is connected in serieswith a load, across the two terminals of a direct voltage source and inwhich an auxiliary circuit is present which includes a seriesarrangement of a first capacitor and a second thyristor. The firstthyristor may be rendered nonconducting with the aid of the auxiliarycircuit in the charged condition of the first capacitor, a chargecircuit of the first capacitor being connected at least partially inparallel with the load.

A known switch arrangement of the kind mentioned above is described, forexample, in United Kingdom Pat. specification No. 995,6l5. In thisswitch arrangement the first capacitor (also called the commutationcapacitor) is charged prior to rendering the first thyristornon-conducting. Subsequently this capacitor is arranged in parallel withthe first thyristor due to the second thyristor being renderedconducting so that the discharge current of the capacitor counteractsthe load current flowing through the first thyristor. The currentflowing through the first thyristor consequently drops below its holdingcurrent value so that this thyristor is cut off.

A drawback of this known switch-arrangement is that when the firstthyristor is rendered non-conducting, the first capacitor is not onlydischarged but even assumes an opposite charge. This means that thiscapacitor cannot be an electrolytic capacitor occupying little space butmust be a bulky capacitor, for example, a paper capacitor.

A further possibility for a switch arrangement would be to use a seriesarrangement of a main thyristor, an auxiliary inductance and the load,while a commutation capacitor would then be connected across theauxiliary inductance through an auxiliary thyristor. In that case avoltage would be generated across the auxiliary inductance due to thecommutation capacitor being discharged across the auxiliary inductance(as a result of the auxiliary thyristor being rendered conducting),which voltage would render the main thyristor nonconducting. Thisfurther possibility for a switcharrangement also has the same drawbackas the one de-- scribed above, namely the capacitor is also recharged toan opposite polarity so that it is not possible to use an electrolyticcapacitor of small dimensions.

The aforesaid drawbacks in the known switch arrangements limits the usethereof in apparatus in which the dimensions of the circuit elements arecritical such as, for example in electronic flash devices forphotography.

An object of the present invention is to provide a circuit arrangementof the kind described in the preamble in which the capacitor(s) is(are)prevented from being charged to an opposite polarity, so that it (they)may have relatively small dimensions.

According to the invention the switch-arrangement includes a thyristorwhich is connected in series with a load across the two terminals of adirect voltage source. An auxiliary circuit is present which includes aseries arrangement of a first capacitor and a second thyristor. Thefirst thyristor may be rendered non-conducting with the aid of theauxiliary circuit in the charged condition of the first capacitor, acharge circuit of the first capacitor being connected at least partiallyin parallel with the load. The switch arrangement is characterized bythe fact that the auxiliary circuit also includes a second capacitorwhich is shunted by a resistor and which is arranged in series with thesecond thyristor.

An advantage of this switch-arrangement is that the second capacitor,which is normally uncharged as a result of the (discharge) resistorconnected thereacross, is charged during the extinguishing process ofthe first thyristor so that this (counter) voltage across the secondcapacitor can reduce the current flowing through the first capacitor tozero before this first capacitor is recharged. In addition the secondcapacitor itself is incorporated in the direct current circuit in such amanner that a recharge of this second capacitor need not be feared. Bothcapacitors may therefore be electrolytic capacitors.

In a preferred embodiment of a switch-arrangement according to theinvention the first capacitor is shunted by a diode. An advantagethereof is that there is greater freedom in the choice of thecapacitance of the second capacitor, which gives rise to the fact thatthe available recovery time for the first thyristor after blocking ofthis thyristor may be increased so that it is easy to remain above theminimum recovery time required for this thyristor.

In a further preferred embodiment the switcharrangement is included in acircuit in which a gas discharge flash tube (electron flash tube) formsthe load and a control circuit of the second thyristor is equipped witha photosensitive element.

The last-mentioned preferred embodiment has the advantage that aflashing unit of small dimensions can be obtained with which flashes canbe effected in known manner, for example, for taking photographs, whilethe flashing duration is determined by the quantity of light which isprojected onto an object to be photographed by the gas discharge flashtube. The blocking command is then derived from the light reflected bythe irradiated object onto the photosensitive element in the circuit ofthe second thyristor. The use of a photosensitive element to terminatethe flash duration is well known in the art and is described, forexample, in U.S. Pat. No. 3,662,213.

In order that the invention may be readily carried into effect, anembodiment thereof will now be described in detail, by way of example,with reference to the accompanying diagrammatic drawing in which thesole FIGURE illustrates a preferred form of the invention.

In this drawing the reference numeral 1 denotes a supply capacitor whichis charged to a relatively high voltage by means of a direct voltagesource consisting of, for example, a voltage converter. A seriesarrangement of a gas discharge flash tube (electron flash tube) 2 and afirst thyristor 3 is connected across the capacitor 1. Connected inparallel with this series arrangement of the tube 2 and the thyristor 3is a series arrangement of a diode 4, a resistor 5 having a high ohmicvalue, a first electrolytic capacitor 6, a resistor 7 having a highohmic value, and a diode 8. Furthermore, the assembly comprisingcapacitor 6, resistor 7 and diode 8 is shunted by a second thyristor 9and a second electrolytic capacitor 10. Furthermore, a dischargeresistor 11 is connected across capacitor 10. lf

necessary, a diode 12 may be connected across capacitor 6.

The operation of the described circuit is as follows: Simultaneouslywith the charging of the supply capacitor l, the commutation capacitor 6is also charged (through the circuit 4, 5, (6), 7, 8). For the timebeing the capacitor is uncharged.

Due to the application of simultaneous pulses on the control electrodesof the tube 2 and the thyristor 3, the capacitor 1 starts to dischargeacross the tube 2. When a desired quantity of the light emitted fromthis tube is reflected through an object (not shown) onto aphotosensitive cell (not shown), which is included in the controlcircuit 13 of the thyristor 9 in a manner described in U.S. Pat. No.3,662,213, the variation in resistance of this photosensitive cellresults in the thyristor 9 being triggered into conduction. Then acurrent starts to flow in the circuit comprising capacitor 6, thyristor9, capacitor 10, so that the current already present in thyristor 3 iscounteracted. As a result the current flowing through thyristor 3 dropsbelow its holding current value whereupon this thyristor is cut-off. Thecapacitor 10 is meanwhile charged to a slight extent and this chargingis also continued through the circuit 2, 6, 12, 9, 10 after thethyristor 3 is cut-off. This further charging of the capacitor 10 andthe simultaneous discharging of capacitor 6 results in a negative anodevoltage at the thyristor 3 during its recovery time.

If the minimum operating voltage of the tube 2 is not considered, thecurrent flowing through the circuit 2, 6, 10 will stop when the vectorsum of the voltages across the capacitors 10 and 6 has become equal tothe voltage of the supply capacitor 1. This means that in the absence ofdiode 12, if the capacitance of capacitor 10 is chosen to be smallerthan that of capacitor 6, the last-mentioned capacitor is certainly notrecharged so that electrolytic capacitors may be chosen for the twocapacitors 6 and 10. The capacitances of the capacitors are alsodetermined by the required recovery time of the thyristor 3, the voltageto which the capacitors 1 and 6 are previously charged, and by theproperties of the load 2.

The thyristor 9, which has been rendered conducting to block thethyristor 3, becomes non-conducting when the current flowing throughthis thyristor 3 is reduced to zero after the capacitor 10 has beencharged. The capacitor 10 is then discharged across the resistor 11 sothat this capacitor is again ready for the next blocking process.

Since the capacitor 6 is not recharged only when the capacitance of thecapacitor 10 is at the most equal to that of the capacitor 6, the normalspread in capacitance of the capacitors must be taken into account whendesigning a practical switch-arrangement.

Thus, either the capacitance of capacitor 10 may be made so small thateven in the most unfavorable case the capacitance of the capacitor 10does not exceed that of the capacitor 6, or the capacitor 6 may beshunted by a diode (12) so that recharging is prevented. When using thediode 12 it is possible to choose a higher capacitance for the capacitor10, for example, equal to that of capacitor 6, so that the time whichelapses before a positive voltage is present again at the anode of thethyristor 3 (positive relative to its cathode) is at a maximum.

The two diodes 4 and 8 serve to prevent direct coupling of the potentialof the capacitor 6 during the blocking process to the potential of oneof the supply terminals.

If desired, the tube 2 and the thyristor 3 may alternatively beinterchanged. In that case one end of the series arrangement ofthyristor 9 and capacitor 10 can be connected to the positive terminalof the direct voltage source and the other end of this seriesarrangement can be connected to the junction of capacitor 6 and resistor7.

What is claimed is:

l. A switching device adapted for connection to the terminals of asource of DC voltage comprising, first and second thyristors, a loadincluding a flash discharge tube, means connecting the first thyristorin series with the load across the terminals of the DC voltage source, aresistor, first and second capacitors, an auxiliary control circuitcoupled to the first thyristor comprising the series arrangement of thefirst capacitor, the second thyristor, and the parallel connection ofthe second capacitor and said resistor, a charge circuit for said firstcapacitor connected in series therewith across the terminals of thevoltage source and arranged to provide a charge path for said firstcapacitor which is exclusive of said first thyristor, means includingsaid auxiliary circuit for turning said first thyristor off in thecharged condition of said first capacitor, and means for applying atrigger voltage to the control electrode of said first thyristor.

2. A device as claimed in claim 1 further comprising a diode connectedin shunt with the first capacitor and poled in the same direction as thesecond thyristor.

3. A device as claimed in claim 1 wherein said charge circuit isconnected in shunt with the series arrangement of the load and the firstthyristor.

4. A device as claimed in claim 3 further comprising a diode connectedin shunt with the first capacitor and with the same polarity as thesecond thyristor.

5. A device as claimed in claim 4 wherein said charge circuit furthercomprises a second diode connected between one terminal of the voltagesource and one terminal of the first capacitor and a third diodeconnected between the other terminal-of the first capacitor and theother terminal of the voltage source, said second and third diodes eachbeing connected with opposite polarity to that of the first diode.

6. A device as claimed in claim 1 wherein the capacitance value of saidsecond capacitor is limited to a range of values that is lower than thecapacitance of the first capacitor and with a maximum value that isequal to that of the first capacitor.

7. A switching device for a flash discharge tube comprising, a source ofDC voltage, first and second thyristors, means connecting said firstthyristor in series with the tube across the terminals of the DC voltagesource, a resistor, first and second capacitors, a charge circuit forthe first capacitor connected in series therewith across the terminalsof the voltage source so as to charge said first capacitor in thecut-off state of said first thyristor, means serially connecting saidfirst capacitor, said second thyristor and the parallel connection ofthe second capacitor and said resistor across said first thyristorwhereby conduction in said second thyristor is operative to cut-offconduction in the first thyristor, and means for selectively applyingtrigger signals to the control electrodes of said-first and secondthyristors.

8. A device as claimed in claim 7 wherein the capacitance of said secondcapacitor is less than or at the most equal to the capacitance of saidfirst capacitor.

9. A device as claimed in claim 7 further comprising a photosensitiveelement connected to the control electrode of the second thyristor andarranged to be irradiated by the light reflected by an object exposed tothe a diode connected in parallel with said first capacitor. 5 flashtube light- 10. A device as claimed in claim 9 further comprising 133 33UNITED STATES PATENT oTTTcT:

(IER'EEFICA'IE ()il CURREQTN Patent No. 3,740,610 Dated June 19, 1973Inventor s) GERHARD RONCKE It is certified that error appears in theaboveident ified patent and that said Letters Patent are herebycorrected as shown below:

col 1 line 8, cancel the comma 7 line 9,, cancel "and in" and insert aperiod 7 line 10, cancel "which an" and insert An line 19, cancel"specification";

line 50, cancel "an" and insert the col. 2, line-l2, cancel "this" andinsert the -7 line 57 after "converter" insert (not shown) col, 3, line45, cancel "3" and ins ert 9 Signed and sealed tl'lis 27th day ofNovember 1973.

(SEAL) I Attes-t'4' I v v EDWARD M.FLETCHER,JR.' RENE D, TEGTMEYERAttesting Officer Acting Commissioner of Patents

1. A switching device adapted for connection to the terminals of asource of DC voltage comprising, first and second thyristors, a loadincluding a flash discharge tube, means connecting the first thyristorin series with the load across the terminals of the DC voltage source, aresistor, first and second capacitors, an auxiliary control circuitcoupled to the first thyristor comprising the series arrangement of thefirst capacitor, the second thyristor, and the parallel connection ofthe second capacitor and said resistor, a charge circuit for said firstcapacitor connected in series therewith across the terminals of thevoltage source and arranged to provide a charge path for said firstcapacitor which is exclusive of said first thyristor, means includingsaid auxiliary circuit for turning said first thyristor off in thecharged condition of said first capacitor, and means for applying atrigger voltage to the control electrode of said first thyristor.
 2. Adevice as claimed in claim 1 further comprising a diode connected inshunt with the first capacitor and poled in the same direction as thesecond thyristor.
 3. A device as claimed in claim 1 wherein said chargecircuit is connected in shunt with the series arrangement of the loadand the first thyristor.
 4. A device as claimed in claim 3 furthercomprising a diode connected in shunt with the first capacitor and withthe same polarity as the second thyristor.
 5. A device as claimed inclaim 4 wherein said charge circuit further comprises a second diodeconnected between one terminal of the voltage source and one terminal ofthe first capacitor and a third diode connected between the otherterminal of the first capacitor and the other terminal of the voltagesource, said second and third diodes each being connected with oppositepolarity to that of the first diode.
 6. A device as claimed in claim 1wherein the capacitance value of said second capacitor is limited to arange of values that is lower than the capacitance of the firstcapacitor and with a maximum value that is equal to that of the firstcapacitor.
 7. A switching device for a flash discharge tube comprising,a source of DC voltage, first and second thyristors, means connectingsaid first thyristor in series with the tube across the terminals of theDC voltage source, a resistor, first and second capacitors, a chargecircuit for the first capacitor connected in series therewith across theterminals of the voltage source so as to charge said first capacitor inthe cut-off state of said first thyristor, means serially connectingsaid first capacitor, said second thyristor and the parallel connectionof the second capacitor and said resistor across said first thyristorwhereby conduction in said second thyristor is operative to cut-offconduction in the first thyristor, and means for selectively applyingtrigger signals to the control electrodes of said first and secondthyristors.
 8. A device as claimed in claim 7 wherein the capacitance ofsaid second capacitor is less than or at the most equal to thecapacitance of said first capacitor.
 9. A device as claimed in claim 7further comprising a diode connected in parallel with said firstcapacitor.
 10. A device as claimed in claim 9 further comprising aphotosensitive element connected to the control electrode of the secondthyristor and arranged to be irradiated by the light reflected by anobject exposed to the flash tube light.